The present invention relates to separation of composite video signals, such as NTSC or PAL signals. In particular, it relates to accurate decoding of chrominance and luminance components, which may reduce so-called dot crawl and false color artifacts of at least some images.
According to Grassman's laws, the human eye can distinguish three kinds of differences or variations. These three components can be represented in many ways. One common way is to represent them as intensities of red (R), green (G), and blue (B) light. They can also be represented as a single luminance (Y) component, along with two chrominance (C) components. The chrominance components can be viewed as color difference signals (B-Y, R-Y), or in polar coordinates as an angle (hue) and magnitude (saturation.)
Typically, composite video signals are generated by adding a baseband luminance signal to a quadrature modulated chrominance signal. Two commonly used standards are known as “NTSC” (developed by the National Television System Committee in the USA in 1953 to be compatible with black-and-white transmissions) and “PAL” (developed in Europe in the 1960s, employing the technique of phase-alternating lines.) There are several variations of these techniques, including the use of different line and frame frequencies and different sub-carrier frequencies used for quadrature modulation. Details of such techniques are well known and can be found in the relevant national standards, or in international standards such as ITU-R BT.470. The theory about the components of visible light and the techniques used for generation of composite video signals are also well known and can also be found in textbooks on video signal processing, such as Digital Video and HDTV Algorithms and Interfaces, by Charles Poynton.
The simplest way to separate the luminance and chrominance components of a composite video signal is to use a combination of low-pass and high-pass filters before quadrature demodulation. This technique assumes that the majority of the luminance signal is below a certain frequency, while the majority of the chrominance signal is above the same frequency. Because such filters only operate in the horizontal direction of the image, this technique is considered one-dimensional. A block diagram of a device employing this technique appears in FIG. 1. The composite signal 100 goes through a low-pass filter 101 to generate the luminance signal 102. The composite signal 100 also goes though a high-pass filter 103 to generate the modulated chrominance signal 104. The modulated chrominance signal 104 is then demodulated by a quadrature demodulator 105 to form the color difference signals 106, 107.
A disadvantage of this configuration is that, in practice, there is some overlap between luminance and chrominance signals in the frequency domain. Therefore, this structure will cause some luminance information to be decoded as chrominance, and vice versa, resulting in visible artifacts in the decoded image. The restriction of the luminance bandwidth to lower frequencies also results in a lower quality image.
Another way of separating luminance and chrominance recognizes that the luminance and chrominance signals typically do not change a great deal between adjacent lines, and the chrominance subcarrier is designed to have opposite phase for either adjacent lines (NTSC) or every second line (PAL.) Thus, by averaging the composite signal for two lines, the modulated chrominance signal will cancel, leaving only the luminance signal. By taking the difference of two lines, the luminance signal will be cancelled, leaving only the chrominance signal. This type of structure is known as a comb filter. To better understand the operation of the comb filter, it is useful to consider the power spectrum of the composite video signal. FIG. 13 from Multimedia —Video Signals—by Philipp Sluallek and Marco Lohse, accessed at http://graphics.cs.uni-sb.de/Courses/ss03/Multimedia/folien/Video.pdf on 16 May 2005, illustrates the overlap between chrominance sidebands and luminance sidebands in an NTSC color coded signal. A similar illustration can be found on p. 362 of the previously referenced book by Charles Poyton. One practicing this art will recognize that the chrominance information is quadrature modulated by a subcarrier whose frequency is an odd multiple of one half the video line rate. This relationship causes the luminance and chrominance power to have peaks that are interleaved in the frequency domain. A filter that averages the composite signal for two lines has a frequency response that resembles the teeth of a comb, passing the luminance peaks, and rejecting the chrominance peaks. Because this structure operates in both horizontal and vertical directions of the image, it is considered a two-dimensional technique. A block diagram of a structure employing a simple comb filter appears in FIG. 2. The composite signal 100 goes through a delay element 201, which delays the signal by one (NTSC) or two (PAL) lines. The delayed signal 202 goes to an adder 203, where it is added to the composite signal 100 to obtain the luminance component 204. The delayed signal 202 also goes to a second adder 205, where it is subtracted from the composite signal 100 to obtain the modulated chrominance component 206. Scale factors may be required before or after the adders to ensure that the component signals are in the correct range. The modulated chrominance signal 206 is then demodulated by a quadrature demodulator 207 to form the color difference signals 208, 209.
One problem with the simple comb filter structure of FIG. 2 is that the assumption that luminance and chrominance signals do not change substantially between lines is not always true. Therefore, the decoded image will have visible artifacts around horizontal (line-to-line) transitions. Another disadvantage of this structure is that it is sensitive to errors in the phase of the chrominance signal. If the phase of the chrominance subcarrier is misaligned after the delay, adding the phase shifted signals will result in imperfect cancellation of the chrominance signal.
To address the problem of horizontal transitions, more complicated signal separation devices use signal information from three or more lines, and employ a vertical processing block to detect transitions and select various combinations of the lines based on that detection. The vertical processing block may also select a horizontal filter output when an appropriate combination of lines cannot be found. An example of this type of structure can be found in UK Patent Application GB 20666 15, “Improvements to Color Television Decoding Apparatus.”
Further improvements can be realized by using signal information from multiple lines of the image. Because these structures operate in horizontal and vertical directions of the image, as well as over multiple frames, they are considered three-dimensional. Examples can be found in U.S. Pat. No. 5,473,389, “Y/C Separator Using 3-D, 2-D, and 1-D Filters,” and U.S. Pat. No. 5,502,509, “Chrominance-Luminance Separation Method and Filter Performing Selective Spatial Filter Based on Detected Spatial Correlation.” A generalized block diagram of the enhanced comb filter structure, which may be two-dimensional or three-dimensional depending on whether any of the delay elements store entire frames, appears in FIG. 3. For the sake of generality, the term “vertical processing” will be used herein to include two-dimensional and three-dimensional processing. The composite signal 100 goes to a cascade of delay elements 301, which delay the signal by various multiples of the line or frame period, generating multiple delayed signals 302. Although only two delay elements are shown in the figure, more could be added without departing from the general structure. The delayed signals 302 and the composite signal 100 proceed to a vertical processing block 303, which determines the best combination of signals to generate luminance 304 and modulated chrominance 305 signals. The modulated chrominance signal 305 is then demodulated by a quadrature demodulator 306 to form the color difference signals 307, 308.
In the configurations described above, the signal separation operations occur before quadrature demodulation of the chrominance signal. These can be called “passband” structures. To address the problem of subcarrier phase sensitivity, some signal separation devices perform demodulation before signal separation by the vertical processing block. These can be called “baseband” structures. Examples of baseband structures appear in U.S. Pat. No. 6,052,157, “System and Method for Separating Chrominance and Luminance Components of a Color Television System;” U.S. Pat. No. 6,175,389, “Comb Filtered Signal Separation;” and U.S. Pat. No. 6,459,457, “Adaptive Color Comb Filter.” A generalized block diagram of the baseband comb filter structure appears in FIG. 4. The composite signal 100 goes to a composite delay element 401, which generates a delayed composite signal 402. The composite signal 100 also goes to a quadrature demodulator 403, which generates a complex baseband signal 404. The complex baseband signal 404 goes to a horizontal processing block 405, which generates a filtered complex baseband signal 406. The filtered complex baseband signal 406 goes to a cascade of complex baseband delay elements 407, which delay the complex baseband signal by various multiples of the line or frame period, generating multiple delayed complex baseband signals 408. Although only two baseband delay elements are shown in FIG. 4, more could be added without departing from the general structure. The delayed complex baseband signals 408 and the filtered complex baseband signal 406 proceed to vertical processing block 409, which determines the best combination of signals to generate first color difference signals 410, 411 and second color difference signals 416, 417. The first color difference signals 410, 411 are used for output. The second color difference signals 416, 417 proceed to a remodulator 412, which generates a modulated chrominance signal 413. The second color difference signals 416, 417 may or may not be the same as the first color difference signals 410, 411. The modulated chrominance signal 413 goes to adder 414, which subtracts the modulated chrominance signal 413 from delayed composite signal 402 to form the luminance output 415.
The main disadvantage of the baseband structure configuration is that it requires increased memory space to implement the complex baseband delay elements. Because the baseband signal is complex, it requires twice the memory as the composite signal, assuming the same precision and sampling rate requirements. This is because the composite signal is sampled as a number that has only a real part whereas the complex baseband signal has both real and imaginary parts. This requirement may be reduced by decimating or reducing the precision of the complex baseband signal. For example, see FIG. 10 of U.S. Pat. No. 6,175,389 and the relevant description. However, both decimation and precision reduction result in the loss of signal information that may be useful for later processing. Decimation of the baseband signal also requires that interpolation be done before remodulation, increasing the complexity of the implementation. A device and method to address these shortcomings was disclosed in U.S. patent application Ser. No. 10/725,955, which shares a common inventor with the present application and was assignable to the same company at the time of invention.
In order to avoid unwanted artifacts like dot crawl (caused by chrominance information being decoded as luminance) and false color (caused by luminance information being decoded as chrominance), the vertical processing block employed by the signal separation structures described above requires some type of adaptive behavior in response to the input signal characteristics. A number of proposals have been presented for changing the vertical processing block structure and/or operation in response to transition conditions. The simplest adaptive techniques switch between comb filtering and bandpass filtering based on the result of horizontal or line-to-line transition detection. Examples may be found in the previously mentioned UK Patent Application GB 2066615, as well as in U.S. Pat. No. 4,179,705 and U.S. Pat. No. 4,240,105, both titled “Method and apparatus for separation of chrominance and luminance with adaptive comb filtering in a quadrature modulated color television system.” More complex adaptive techniques use different weighting coefficients to combine delayed and undelayed video signals, depending on the transition detection result. A small set of weighting coefficients may be used, as in the previously mentioned U.S. Pat. No. 6,459,457, or smoothly variable weighting coefficients may be used, as in U.S. Pat. No. 4,864,389, “Comb filter method and apparatus for chrominance and luminance separation in quadrature modulated color subcarrier television systems” and the previously mentioned U.S. Pat. No. 6,175,389.
All these techniques suffer from the fact that horizontal transitions are difficult to detect when there is significant overlap of luminance and chrominance signals. Tradeoffs can be made by adjusting the characteristics of the chrominance and/or luminance signals used for transition detection and/or comb filtering. For example, narrower bandwidth chrominance signals reduce the chance of false color, and wider bandwidth chrominance signals reduce the chance of dot crawl. Existing techniques generally select a tradeoff and vary the weighting of delayed and undelayed signals. Therefore, there exists a need in the art for an adaptive vertical processing technique that can dynamically adjust the characteristics of the luminance and/or chrominance signals, as well as the weighting of delayed and undelayed signals.
An opportunity arises to improve the accuracy of decoding of chrominance and luminance components from composite video signals without substantially increasing processing complexity. Reduced dot crawl and false color artifacts in decoded images may result.